Conifer Establishment and Encroachment on Subalpine Meadows Around Mt

Western Washington University Western CEDAR

WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship

Summer 2020

Conifer establishment and encroachment on subalpine meadows around Mt. Baker, WA

Ben Hagedorn Western Washington University, [email protected]

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Recommended Citation Hagedorn, Ben, "Conifer establishment and encroachment on subalpine meadows around Mt. Baker, WA" (2020). WWU Graduate School Collection. 981. https://cedar.wwu.edu/wwuet/981

This Masters Thesis is brought to you for free and open access by the WWU Graduate and Undergraduate Scholarship at Western CEDAR. It has been accepted for inclusion in WWU Graduate School Collection by an authorized administrator of Western CEDAR. For more information, please contact [email protected]. Conifer establishment and encroachment on subalpine meadows around Mt. Baker, WA

Ben Hagedorn

Accepted in Partial Completion of the Requirements for the Degree Master of Arts

ADVISORY COMMITTEE

Dr. Aquila Flower, Chair

Dr. Andy Bach

Dr. Michael Medler

GRADUATE SCHOOL

David L. Patrick, Dean

Master’s Thesis

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Ben Hagedorn

8/8/20

Conifer establishment and encroachment on subalpine meadows around Mt. Baker, WA

A Thesis Presented to The Faculty of Western Washington University

In Partial Fulfillment Of the Requirements for the Degree Master of Arts

by Ben Hagedorn August 2020

Abstract

The subalpine ecotone is experiencing significant change in habitat availability and connectivity as a result of climate change and climate variability. To understand how these changes vary temporally and spatially in the Pacific Northwest, we collected cross-sections and counted whorls of conifers along four transects around Mt. Baker, Washington in the North Cascades. In addition to the samples collected, we also gathered data on microsite conditions that impact seedling establishment. Using partial correlation analyses, we compared establishment dates to climate variables in five-year bins, and used normal correlation analyses along with other statistical tests to determine the effect of various microsite variables on establishment. Our results show that establishment has occurred in pulses throughout the 20th century, with greater establishment on drier sites during periods of greater precipitation, and greater establishment on wetter sites during periods of higher temperatures. We found that April precipitation and September temperature are particularly strongly correlated with establishment rates, suggesting that the best conditions for conifer seedling establishment occur in years with the warmth and soil moisture needed for the growing season to last longer into late summer. Within individual meadows, conifer seedling establishment was greater on convex surfaces and in areas with a higher percentage of vaccinium cover.

The tree species present in each meadow also play a role in determine the timing of establishment pulses, and the distribution of encroaching seedlings, with silver fir only being able to establish close to its seed source and yellow cedar establishing on drier microsites. Our findings at Mt. Baker show that periods of conifer establishment have occurred somewhat synchronously across multiple mountains in the region, but we also identified distinct spatial and temporal differences linked to local site conditions.

Acknowledgements

I am deeply grateful to all the people who have helped me complete this work as a part of my Master’s

Degree. I would not have been able to do this without the advice and support of my committee chair, Dr.

Aquila Flower, or my committee members Dr. Andy Bach, Dr. Michael Medler, and Dr. Eric DeChaine.

Numerous other faculty and administration staff were helpful, especially Dr. Andy Bunn who helped me trouble-shoot numerous technical and non-technical problems, and Ed Weber who helped me navigate bureaucracy.

My fellow graduate students were incredibly supportive. Particularly the fellow members of the tree ring lab: Hannah LaGassey, Dustin Gleaves, and Chris Trinies. Being able to work with you all, freely bounce ideas off each other, and get some insight into different areas of dendrochronology was easily one of the most enjoyable parts of my time at Western. You all helped make me a better researcher. Kate Welch,

Hannah Drummond, and Elliot Winter were also wonderful and helpful throughout this process. They were always checking in to see how my thesis was going, providing me an outlet to vent and some perspective, as well as useful feedback that drastically improved my skills.

I would not have made it through without my family and friends. Especially my mom and dad, Lin and

Tom Hagedorn, as well as my sister Jenn Hagedorn. They listened to me complain, provided so much encouragement, helped me move twice, and on occasion sent me boxes of food.

Finally, I could not have collected all this data on my own. So many people came out the help me scout for sites and cut down tiny little trees. So thank you to Hannah LaGassey, Lin Hagedorn, Tom Hagedorn, Elliot

Winter, Jameson Goff, Shea Simpson, Mia Henderson, Michael Schroeder, Sean Fitzpatrick, Bill LaGassey,

Rai Dachenhausen, and Colter Lemons.

Table of Contents

Abstract ...... iv

Acknowledgements ...... v

List of Tables and Figures ...... vii

Introduction ...... 1

Chapter 1: Methods ...... 4

Chapter 2: Results ...... 12

Chapter 3: Discussion ...... 32

Chapter 4: Conclusion ...... 45

Works Cited ...... 48

Appendix A ...... 53

Appendix B ...... 55

Appendix C ...... 57

List of Tables and Figures

Figure 1- Study area map Page 5 Figure 2- Study area climograph Page 6 Figure 3- Study area snowpack graph Page 6 Figure 4- Transect microtopography image Page 8 Figure 5- Study area establishment Page 13 Figure 6- Transect establishment Page 14 Figure 7- Study area partial annual and seasonal correlations Page 16 Figure 8- Scott Paul partial annual and seasonal correlations Page 17 Figure 9- Grouse Ridge partial annual and seasonal correlations Page 18 Figure 10- Scott Paul partial monthly temperature correlations Page 19 Figure 11- Grouse Ridge partial monthly temperature correlations Page 19 Figure 12-Scott Paul partial monthly precipitation correlations Page 20 Figure 13- Grouse Ridge partial monthly precipitation correlations Page 20 Figure 14- Scott Paul partial monthly snowpack correlations Page 21 Figure 15- Grouse Ridge partial monthly snowpack correlations Page 21 Figure 16- Scott Paul partial monthly snowfall correlations Page 22 Figure 17- Grouse Ridge partial monthly snowfall correlations Page 22 Figure 18- Visualization of spatial relationships at all transects Page 24 Figure 19- Microtopography and age Page 27 Figure 20- Seed source distance and age Page 28 Figure 21- Seed source distance and age at each transect Page 29 Figure 22- Tree growth and age Page 30 Figure 23- Snowpack at Mt. Baker and Mt. Rainier Page 37 Figure 24- Regional establishment pulses Page 39 Figure A1- Climate variables correlations page 53 Figure B1- Residual plots Page 56 Figure C1- Scott Paul normal annual and seasonal correlations Page 57 Figure C2- Grouse Ridge normal annual and seasonal correlations Page 57 Figure C3- Scott Paul normal monthly temperature correlations Page 58 Figure C4- Grouse Ridge normal monthly temperature correlations Page 58 Figure C5-Scott Paul normal monthly precipitation correlations Page 59 Figure C6- Grouse Ridge normal monthly precipitation correlations Page 59 Figure C7- Scott Paul normal monthly snowpack correlations Page 60 Figure C8- Grouse Ridge normal monthly snowpack correlations Page 60 Figure C9- Scott Paul normal monthly snowfall correlations Page 61 Figure C10- Grouse Ridge normal monthly snowfall correlations Page 61

Table 1- Transect coordinates and elevation Page 5 Table 2- Climate variables tested Page 10

vii7

Table 3- Spatial variables tested Page 11 Table 4- Ecological variables tested Page 11 Table 5- Samples by crossdating method Page 12 Table 6- Interseries correlations by transect Page 12 Table 7- Mean sensitivity by transect Page 12 Table 8- Density and microtopography Wilcoxon test values Page 23 Table 9- Density and distance correlation values Page 23 Table 10- Density and vegetation correlation values Page 24 Table 11- Age and microtopography Wilcoxon test values Page 27 Table 12- Age and distance correlation value page 28 Table 13- Age and growth correlation values Page 30 Table 14- Results from studies at Paradise Page 41 Table A1- Climate variable correlation values Page 54 Table B1- Shapiro-Wilkes results Page 55 Table B2- Fligner-Killeen results Page 55

viii8

Introduction

Subalpine meadows play an important ecological role both within and beyond the subalpine ecotone. In western North America, meadows are key summer feeding grounds for native cervids and bovids, and serve as the sole habitat for several endangered species, and species sensitive to climate change such as the American pika and the Olympic marmot (Lubetkin et al., 2017; Armitage, 2013; Erb et al., 2011).

Subalpine meadows store large amounts of snowmelt and release it slowly throughout the summer, which helps regulate stream flows in the Pacific Northwest (Lubetkin et al., 2017; Clow et al., 2003). The wildflowers in meadows also serve as a critical food source for pollinators (Rolland and Matter, 2007). In many subalpine ecosystems, treelines are moving upslope and trees are moving into previously treeless meadows as a result of warming global temperatures (Malanson et al., 2007). The fragmentation and net loss of meadow habitat has a cascading effect throughout the ecosystem. A reduction in water storage capacity will lead to increased stream temperature and greater variability in stream flow, both of which are detrimental to salmon species (Clow et al., 2003; Mantua et al., 2009). The loss of forage and habitat can lead to increased stress on species that are already seeing significant declines, such as mountain goats and many pollinators (Rolland and Matter, 2007; Wells et al., 2011).

The cascading effects of climate change on subalpine meadow ecosystems are still not well understood. Little is known about the spatial and temporal variability of changes occurring in western

North America’s subalpine meadows. Previous research has been conducted in only a few specific locations, primarily in national parks (Whiteside and Butler, 2011). In Washington State, there have been several studies conducted on conifer encroachment and treeline advance around Mt. Rainier, but very few on other mountain peaks in the Cascade Mountains. The timing of conifer encroachment is variable across the Cascade Mountains, in part because the climatic controls on seedling establishment vary depending on local climatic conditions. Areas that are wetter tend to have more conifers establish when conditions are

1 warmer and drier, and areas that are drier tend to have more conifer establishment when conditions are cooler and wetter (Franklin et al., 1971; Woodward et al., 1995; Rochefort and Peterson, 1996; Miller and

Halpern, 1998; LaRoque et al., 2000; Flower et al 2017). Wetter areas tend to be on west facing slopes that receive increased precipitation due to orographic uplift, and on north facing slopes that receive less solar radiation. Drier areas are often on south and east facing slopes, which get more solar radiation or are in a rain shadow. Microsite characteristics can lead to nearby areas having different temporal patterns of conifer establishment, even when these areas experience the same landscape scale climate patterns (Franklin et al.,

1971; Rochefort and Peterson, 1996; Zald et al., 2012; Flower et al 2017). Microsite characteristics also drive spatial pattens of establishment. Meadows throughout the region show patchy areas of establishment, with denser establishment on convex, more densely vegetated surfaces regardless of where in the meadow those areas occurred (Franklin et al., 1971; Rochefort and Peterson, 1996; Flower et al 2017). While these patterns have been studied in the southern Cascades as well as in coastal areas of Washington and British Columbia, relatively little work has been done in the North Cascades. Research around Mt. Baker will help fill in a spatial gap in the data, and further examine similarities and differences in patterns throughout the region.

Conifer encroachment has been studied at multiple scales throughout the Pacific Northwest, and there are several landscape scale and microscale patterns of establishment throughout the region. Our study applies newer methods to update and expand the research around Mt. Baker and answer the following questions: 1

1. What are the spatial and temporal patterns of conifer establishment in subalpine meadows on Mt. Baker?

2. How are temporal patterns of conifer establishment related to variability in temperature, precipitation, and snowpack?

3. How are spatial patterns of conifer establishment related to spatial variation in microtopography and plant cover?

4. How do the spatiotemporal patterns of establishment and the associations of those patterns with climate and spatial variables differ between tree species?

5. How similar are the patterns of conifer encroachment and establishment at Mt. Baker to patterns of establishment and encroachment in other parts of the Washington Cascades?

Chapter 1

Methods

1.1 Study Area

Mt. Baker is a heavily glaciated stratovolcano in the North Cascades of Washington State (fig 1). Glaciers on Mt. Baker have advanced and retreated throughout the late Pleistocene and Holocene, with the most recent maximum extent occurring during the mid-1800s (Osborn et al., 2012). Subalpine meadows occur on variable terrain created by older glaciations. The soils underlying these meadows have a base of diamicton, followed by volcanic tephra, silt, sand, and sandy to muddy peat (Osborn et al., 2012).

The subalpine meadows on Mt. Baker experience warm, dry summers and cool, wet winters, on the boundary between two Koppen Climate Classifications: Warm Summer Mediterranean and Temperate

Oceanic (Peel, et al., 2007) (fig 2). Cool, wet winters produce a large amount of snowfall (fig 3). Mt. Baker Ski

Area averaged 1,581 cm of snow per season from 1971 through 2000, and holds the record for most snowfall in snowfall season, totaling 2,850 inches from July 1998 to June 1999 (Leffler et al., 2001).

Figure 1. Study site locations. The two transects along the Scott Paul (SP) Trail are both south facing, and the two transects on Grouse Ridge (GR) are both north Facing.

Site ID Latitude Longitude Elevation (meters) SP West 48.72416 -121.822 1,489 SP East 48.72833 -121.812 1,528 GR West 48.78807 -121.926 1,500 GR East 48.78975 -121.921 1,521 Table 1. Coordinates and elevation for each transect.

Figure 2. Climograph with precipitation and temperature for both the Scott Paul and Grouse Ridge site.

Figure 3. The depth of spring and summer snowpack at each transect.

Trees and other plants growing in the subalpine ecotone are adapted to these cold, wet winter conditions and relatively young soils. The most common tree species at our sites are mountain hemlock

(Tsuga mertensiana), Pacific silver fir (Abies amabilis), and, at the Grouse Ridge site, Alaskan yellow cedar

(Callitropsis nootkatensis). Pacific silver fir tends to be more shade tolerant than mountain hemlock, while mountain hemlock tends to be more tolerant of colder temperatures, and both have similar tolerances to snowpack depth and soil moisture (Woodward et al. 1995; Klinka et. al., 1992). Yellow cedar is more drought tolerant than other tree species in this ecosystem. Under most conditions it is out-competed by faster-growing Pacific silver fir and mountain hemlock which specialize in growing in cold and wet environments, but on drier microsites yellow cedar can outcompete these species (Laroque and Smith, 1999;

Antos and Zobel, 1986). The majority of low-growing meadow plants at our study sites belong to two genera of family Ericaceae: vaccinium and phyllodoce. Common species include pink mountain-heather

(Phyllodoce empetriformis), yellow mountain-heather (Phyllodoce glanduliflora), black huckleberry (Vaccinium membranaceum), and dwarf bilberry (Vaccinium caespitosum). Other notable species are White-flowered rhododendron (Rhododendron albiflorum) and arctic lupine (Lupinus arcticus).

1.2 Field Methods

We collected samples at two sites with two transects per site. One site was on a north facing slope, the other a south facing slope (fig 2). All transects were at a similar elevation, with a maximum difference in elevation of 39 meters (table 1). Each of our four total transects were 50 meters long and two meters wide.

Each transect started near a stand of mature trees that could act as a seed source, and then crossed the meadows from east to west, maintaining elevation across the slope. We divided the transects into a series of square meter plots on either side of the transect, and recorded plot characteristics and tree measurements in each plot.

For every plot, we determined whether the relative local microtopographic position of the plot was concave or convex. The meadows are comprised of both convex and concave features caused by various mass wasting or other erosional processes. If a plot was located on a convex feature in relationship to the topography of the meadow, then that plot was defined as convex (fig 4). We also estimated the percent of the ground covered by phyllodoce, lupinus, and vaccinium.

Figure 4 shows part of the Grouse Ridge West transect. The plots in the low area where Elliot and Sean are gathering data are considered concave. If the transect were to extend further, the plots that would occur on the ridge would be considered convex.

In addition to the plot level data, we collected measurements from every tree along the transect.

We recorded the species and basal diameter for each tree, and we recorded the height of trees when the top of the tree was within arm’s reach. We also recorded the diameter at breast height for all trees tall enough to have one. We collected cross-sections in alternating plots along the transect. We cut the trees as close to the root collar as possible, sometimes digging with a shovel, taking care to cut perpendicular to the direction of growth. For trees under one centimeter in basal diameter, we counted whorls instead of cutting a cross-section to get an estimate of age (Urza and Sibold, 2013).

1.3 Lab Methods

We sequentially sanded cross-sections with a belt sander, using 150-, 220-, and 320-grit sandpaper. We then hand-sanded samples with 400- and 600-grit sandpaper in order to clearly see individual cells. Once

8 sanded, we counted the rings under a dissecting microscope to determine the establishment date of each sample using the Yamaguchi list method for visual cross dating (Yamaguchi, 1991). We then measured ring widths using a Velmex Unislide and statistically cross-dated samples greater than 50 years old using

COFECHA (Speer, 2012; Holmes, 1983). Samples with low inter-series correlations or identifiable dating inaccuracies were measured again and corrected if necessary.

1.4 Statistical Methods

Spatial scale plays an important role when attempting to understand the interactions between establishment and different variables. Therefore, we made sure to conduct our various analyses at the most appropriate spatial scale. For our climate variables we used two different spatial scales: the whole study area, and two sites comprised of two transects each. With this stratified system we can look at the differences between the two sites, and between the general patterns at Mt. Baker and the patterns found in the rest of the Washington Cascades.

All climate data used for this study are PRISM data statistically downscaled using ClimateWNA (Daly et al., 2002; Mitchell and Jones, 2005), as there are no suitable SNOTEL or weather stations near the study area. Snowpack and snowfall data were derived from a monthly water balance model using PRISM data

(Hostetler and Alder, 2016). This model specifically performs well along the Pacific coast, simulating snowpack with a bias of +/-50mm at 68% of 713 SNOTEL sites (Hostetler and Alder, 2016). To address potential errors in temporal accuracy of tree establishment dates, we grouped our establishment dates into five-year bins and we averaged climate data into the same five-year bins and (Flower et al., 2017).

To answer our research question “how do changes in temperature, precipitation, and snowpack relate to temporal patterns of conifer establishment?”, we tested for relationships between the number of trees established in each five-year bin and climate variables using a series of Kendall’s correlation analyses (table

2). Partial correlation analysis is useful when multiple variables have known relationships with each other,

9 such as the relationship between temperature and precipitation, that may confound their apparent correlation with a third variable (Figure A1 and Table A1). We used Kendall’s correlations because our data were neither homoscedastic nor normally distributed (Table B1 and B2). For all statistical tests we used a significance level of 0.05.

Climate Variables – Partial Correlation with Establishment Bin Mean annual temperature (MAT) controlled for MAP Mean annual precipitation (MAP) controlled for MAT Mean growing season temperature (MGST) controlled for MGSP Mean growing season precipitation (MGSP) controlled for MGST Mean water year temperature (MWYT) controlled for MWYP Mean water year precipitation (MWYP) controlled for MWYT Mean monthly temperatures controlled for monthly precipitation Mean monthly precipitation controlled for monthly temperature March-July monthly snowpack depth controlled for monthly temperature and precipitation April, May snowfall controlled for monthly temperature and precipitation Frost-free period (FFP) controlled for MGSP Number of frost-free days (NFFD) controlled for MGSP Table 2. Climate variables used in partial correlation analyses with number of trees established in each 5-year bin. The growing season is defined as the months of May through September.

To answer our research question “how does spatial variation in microtopography and plant cover relate to spatial variability in conifer establishment patterns?”, we used non-parametric tests to assess the relationship between conifer stem density and spatial variables. We used the Wilcoxon rank sum test to determine whether there was significantly greater stem density in either convex or concave plots, and we used Kendall’s correlation analyses to determine if there was a relationship between stem density and percent phyllodoce cover, vaccinium cover, total vegetation cover, and the distance from seed source (table

3). When testing the relationship between stem density and distance from seed source, we also performed the test with just the stem density in convex plots to eliminate any potential influence that microtopography might have.

Spatial Variables – Evaluated with Stem Density Microtopographic position Percent vaccinium cover Percent heather cover Percent vegetation cover Distance from seed source Table 3. Variables used to test relationships between conifer stem density and locations within meadows.

Finally, we tested for relationships between the age and spatial distribution of trees, as well as age and growth characteristics of trees (table 4). We used the Wilcoxon rank sum test to determine whether trees were significantly older in concave or convex plots, and used Kendall’s correlations to determine if there was a relationship between the age of trees and the distance from seed source. We also used Kendall’s correlations to determine if there was a relationship between tree age and basal diameter as well as age and height. We used a logarithmic transformation on tree heights and basal diameters due to a trend in the residuals (fig. A1).

Ecological Variables – Evaluated with Tree Age Microtopographic position Distance from seed source Tree height (log transformation) Tree basal diameter (log transformation) Table 4. Variables used to test relationships in the age of trees. The variables include both spatial and growth variables of trees.

To answer our question, “how do the spatiotemporal patterns of establishment and the associations of those patterns with climate and spatial variables differ between tree species?” we completed the tests described above with all species grouped together, and again individually for each species present. While the tree species do have similar climatic relationships, it is necessary to break it down to the species level to fully understand any differences.

Chapter 2

Results

We collected samples from and determined the ages of 130 mountain hemlock, 65 Pacific silver fir, and 16 Alaska yellow cedar trees, saplings, and seedlings. We were able to successfully crossdate our series using a combination of visual and statistical crossdating (table 5). However, our mean interseries correlations are very low (tables 6 and 7). These low interseries correlations are attributable to a high degree of spatial variability in microsite conditions and pervasive reaction wood in most samples caused by repeat snow events bending small saplings.

Statistically Crossdated Visually Crossdated Whorl Counted All transects 37 84 90 Scott Paul West 14 37 12 Scott Paul East 5 16 42 Grouse Ridge West 10 18 35 Grouse Ridge East 8 13 1 Table 5. The number of samples dated by each dating method

All species R- TSME R-value ABAM R-value CANO R-value value All Transects 0.178 0.18 0.095 0.148 Scott Paul West 0.208 0.212 0.126 NA Scott Paul East 0.113 0.113 NA NA Grouse Ridge West 0.11 0.159 0.038 0.24 Grouse Ridge East 0.128 0.059 NA 0.145 Table 6. Mean interseries ring-width correlations for each species at each transect, all species combined at each transect, and each species at all transects combined.

All species mean TSME mean ABAM mean CANO mean sensitivity sensitivity sensitivity sensitivity All transects 0.408 0.419 0.395 0.358 Scott Paul West 0.408 0.412 0.421 NA Scott Paul East 0.459 0.408 NA NA Grouse Ridge West 0.381 0.399 0.383 0.325 Grouse Ridge East 0.4 0.42 NA 0.368 Table 7. Mean sensitivity of ring-width indices for each species at each transect, all species combined at each transect, and each species at all transects combined

2.1 Establishment Pulses

When looking at the temporal patterns of establishment for all transects combined, there are clear pulses in establishment (fig 5). The first pulse starts in 1926-1930 and ends in 1956-1960. The second pulse starts in 1981-1985 and continues until 2011-2015. The pulses for the individual species are slightly different, but they follow the same general pattern with the exception of yellow cedar, which has no pulses of establishment. Mountain hemlock has establishment pulses from 1926-1930 to 1956-1960 and again from

1981-1985 to 2006-2010. Silver fir has pulses of establishment from 1941-1945 to 1951-1955 and again from

1991-1995 to 2011-2015.

Figure 5. Total number of trees of each species that established in each five-year establishment bin for all the transects combined. The dates on the graph are the start dates of each five-year bin.

The individual transects also show pulses of establishment, except for Grouse Ridge East (fig 6). Pulses at the remaining transects may not exactly replicate the pulses from the combined data, but they do all have similarities to the overall trend. Scott Paul West and Grouse Ridge West both show two pulses: one centered

13 in the 1940s and 1950s and another centered in the 2000s. Scott Paul east shows the pulse in the 2000s, but it doesn’t show any pulse earlier in the 20th century.

Figure 6. The number of trees established in each five-year bin at each individual transct. The dates on the graph are the start dates of each five-year bin.

2.2 Climate Relationships

The most appropriate scale of analysis for climate correlations is at the site level. While the study area does have a common climate, there are distinct differences in temperature, precipitation (fig 2), and snowpack (fig 3) between the Scott Paul and Grouse Ridge sites.

Figure 7. Tau values for partial Kendall’s correlations between climate variables and tree establishment in five-year establishment bins for the whole study area. *=p-value < 0.05, **=p-value < 0.01. Climate variables include Mean Annual Temperature (MAT), Mean Growing Season Temperature (MGST), Mean Annual Precipitation (MAP), Mean Growing Season Precipitation (MGSP), Mean Water Year Temperature (MWYT), Mean Water Year Precipitation (MWYP), Frost Free Period (FFP), and Number of Frost Free Days (NFFD).

Across the whole study site, it is clear that during warmer periods there is an increase in establishment

(fig 7). This is true for annual temperature, growing season temperature, and water year temperature. This pattern is also shown in the length of the frost-free period or total number of frost- free days. While the seasonal and annual relationships are the same for temperature, there are two sperate responses for precipitation. There is a weak positive correlation between annual precipitation and establishment, and there is a weak negative correlation between growing season precipitation and establishment. Snowpack and snowfall are included in sperate graphs due the importance of timing and ease of visualization.

Studying these same variables at the site level can show the differing climate relationships at both sites. Conifer establishment at Scott Paul tends to be more temperature driven, with strong positive

16 relationships for both annual and growing season temperature (fig 8). Conifer establishment at Grouse

Ridge is more precipitation driven with a strong positive relationship for mean annual precipitation (fig 9).

These relationships remain consistent when viewing the normal Kendall’s correlations as opposed to the partial Kendall’s correlations (fig C1 and C2)

Figure 8. Tau values for partial Kendall’s correlations between climate variables and tree establishment in five-year establishment bins for the Scott Paul site. *=p-value < 0.05, **=p-value < 0.01. Mean Annual Temperature (MAT), Mean Growing Season Temperature (MGST), Mean Annual Precipitation (MAP), Mean Growing Season Precipitation (MGSP), Mean Water Year Temperature (MWYT), Mean Water Year Precipitation (MWYP), Frost Free Period (FFP), and Number of Frost Free Days (NFFD).

Figure 9. Tau values for partial Kendall’s correlations between climate variables and tree establishment in five-year establishment bins for the Grouse Ridge Site. *=p-value < 0.05, **=p-value < 0.01. Mean Annual Temperature (MAT), Mean Growing Season Temperature (MGST), Mean Annual Precipitation (MAP), Mean Growing Season Precipitation (MGSP), Mean Water Year Temperature (MWYT), Mean Water Year Precipitation (MWYP), Frost Free Period (FFP), and Number of Frost Free Days (NFFD).

The monthly variables show consistent patterns in differences between the Scott Paul and Grouse

Ridge Sites, but also highlight some key similarities. Scott Paul has a consistent positive relationship between establishment and mean monthly temperature. The strongest and most significant relationship is between establishment and September mean temperature (fig 10). Grouse ridge has a much more inconsistent and generally weaker correlation between establishment and mean monthly temperature, but the strongest relationship there is also in September (fig 11). For monthly precipitation, Scott Paul has weaker, less consistent correlations (fig 12), and Grouse Ridge has stronger consistently positive correlations (fig 13). For both sites there was a strong positive correlation is in April, followed by the strongest negative correlation in May. The pattern of consistent positive correlations for monthly temperature at Scott Paul and precipitation at Grouse Ridge matches the pattern observed in the seasonal and annual variables, but gives more detail about how weather patterns in specific parts of the year or season can have a stronger impact on establishment.

Figure 10. Partial Kendall’s correlation between mean monthly temperature and establishment at the Scott Paul site. *=p-value < 0.05, **=p-value < 0.01.

Figure 11. Partial Kendall’s correlation between mean monthly temperature and establishment at the Grouse Ridge site*=p-value < 0.05, **=p-value < 0.01

Figure 12. Partial Kendall’s correlation between mean monthly precipitation and establishment at the Scott Paul site. *=p-value < 0.05.

Figure 13. Partial Kendall’s correlation between mean monthly precipitation and establishment at the Grouse Ridge site.. *=p-value < 0.05.

When looking at the relationship between establishment and snow variables, there is a continuation of the pattern seen with precipitation. The transition from April to May is critical for establishment at both sites. While it may not be as strong as going from consistently positive to consistently negative, there is a reduction in the strength of the correlation between snowpack and establishment as

20 well as snowfall and establishment at both sites. The pattern is more noticeable at Scott Paul (fig 14) and less noticeable at Grouse Ridge due to the presence of yellow cedar (fig 15). The same can be said for snowfall at Scott Paul (fig 16) and snowfall at Grouse Ridge (fig 17).

Figure 14. Partial Kendall’s correlation controlled for temperature between mean monthly snowpack and establishment at Scott Paul. None of these correlations are significant.

Figure 15. Partial Kendall’s correlation controlled for temperature between snowpack and establishment at Grouse Ridge. *=p-value < 0.05. July is not included for snowpack at Grouse Ridge because there is rarely any snowpack at Grouse Ridge in July.

Figure 16. Partial Kendall’s correlation controlled for temperature between snowfall and establishment at Scott Paul. *=p-value < 0.05.

Figure 17. Partial Kendall’s correlation controlled for temperature between snowfall and establishment at Grouse Ridge. *=p-value < 0.05.

2.3 Spatial Patterns

While the temporal relationships between establishment and climate varied somewhat between sites, the spatial patterns were more consistent (figure 18). We found that spatial relationships were significant at both the transect and study area scale of analysis.

The most consistent relationship was between density of established conifers and microtopography, with significantly denser establishment on convex plots compared to concave plots for all tree species present (table 8). While the species have similar relationships with microtopography, there are differences when it comes to distance from seed source. There is no significant relationship between density of hemlock establishment and distance, but the density of silver fir decreases as distance from seed source increases

(table 9). We confirmed this result by running the same correlation using only convex plots. All the transects started on convex plots and we wanted to eliminate the possibility of a correlation with microtopography instead of distance. We confirmed our previous finding.

There are also significant differences between the type of vegetation coverage and the relationship to establishment. While the combined percentage of heather and vaccinium does not significantly correlate with establishment, when we separated the two cover types we found no correlation between heather and establishment, and a significant but weak positive correlation with vaccinium. This pattern was consistent between all transects (Table 10).

Microtopography-Wilcoxon Test Concave N Convex N W P-Value 128 279 13,808 4.20e-08 Table 8. The number of seedlings established in concave and convex plots

Distance-Kendall’s Correlation All species Z Tau P-Value -2.290 -0.111 0.003 Mountain Hemlock Z Tau P-Value 1.073 -0.042 0.283 Silver Fir Z Tau P-Value -6.203 -0.252 5.545e-10 Table 9. Kendall’s correlation between the density of trees established and distance from seed source for the whole study site.

Vegetation-Kendall’s Correlation Phyllodoce Z Tau P-Value -0.880 -0.003 0.379 Vaccinium Z Tau P-Value 6.687 0.265 2.277e-11 Table 10. Kendall’s correlation between the density of establishment and different vegetation cover types for the whole study area.

Figure 18. Spatial variables at each individual transect. Each grid cell corresponds to a one square meter plot on a transect. Completely white squares in vegetation plots at Grouse Ridge West indicate missing data due to error in the collection process.

2.4 Age patterns

The age range of seedlings sampled using whorl counts is from three to 43 years, while the age range of trees sampled using cross-sections was from 36 to 151 years. The median age of all samples combined is

47 years and the mean is 47.2 years. There are several interesting patterns to note as they relate to the age of the tree sampled. Spatially within the meadows there are no significant differences in how old the trees are in certain parts of the meadows compared to other parts of the meadow. There was no significant relationship between the age of trees in convex plots compared to concave plots (fig 19) (table 11), although the oldest trees were found only on convex surfaces and convex surfaces had an older average age of trees.

There was a statistically significant correlation between age and distance from seed source, but it was a very weak correlation (fig 20) (table 12). When looking at the individual transects some show significant

26 positive correlations, some show significant negative correlations, and some show no significant relationship at all (fig 21).

Microopography and age -Wilcoxon Test Concave N Convex N W P-value 57 150 3910 0.147 Table 11. The results of Wilcoxon Rank Sum test, with a test statistic of W, to determine if there were significantly older trees established in concave or convex plots.

Figure 19. The difference in the age of trees between concave and convex plots.

Figure 20. The relationship between tree age and distance from seed source for the whole study area.

Distance-Kendall’s Correlation Z P-value tau Study Area 2.328 0.02 0.111 Scott Paul West 4.359 1.306e-05 0.397 Scott Paul East -3.221 0.001 -0.287 Grouse Ridge West -0.292 0.771 -0.026 Grouse Ridge East -0.763 0.445 -0.118 Table 12. The relationship between tree age and distance from seed source the study area and each transect.

Figure 21. The relationship between tree age and distance from seed source at the individual transects.

There are signifcant patterns in the relationship between tree measurements and the age of the tree.

We had to use a logarithmic transformation on the basal area and the height of the trees, because after running the correlation without the log transformation there was a still a pattern in the residual plot (fig

A1). There are strong positive correlations between the age of trees and both their height and basal diameter

(fig 22) (table 13).

Figure 22. The relationship between log of basal diameter and tree age as well as log of height and tree age.

Kendall’s Correlation- log Height Z P-value tau 14.473 < 2.2e-16 0.682 Kendall’s Correlation- log Basal Diamter Z P-value tau 14.925 < 2.2e-16 0.712 Table 13. Kendall’s correlations between growth variables and age.

2.5 Individual Species responses

Each tree species responds slightly differently to the same set of conditions. We have noted some of the distinct responses by species throughout the results section, because it is hard to interpret the overall patterns without mentioning some of the most prominent species-specific patterns. Examples of this include: the strong relationship between yellow cedar and snowpack (fig 15) and snowfall (fig 17), and the relationship between silver fir and seed source distance (table 9). There are more interesting and nuanced relationships to note as well, particularly with regards to climate.

Not only are there different patterns in temperature and establishment correlations between Scott

Paul and Grouse ridge, the species that are most responsive to temperature change as well. At Scott Paul, mountain hemlock is the tree species more strongly correlated with temperature (fig 8), but at Grouse ridge silver fir has a stronger correlation (fig 9). Mountain Hemlock establishment correlation with snowpack at both Scott Paul (fig 14) and Grouse Ridge (fig 15) follows a similar trend. There is a general decrease in

30 correlation strength from March through May with a slight increase again in June. Silver fir has a different trend from mountain hemlock and is different at both Scott Paul and Grouse Ridge. At Scott Paul there is a sharp decrease in the correlation between April and May, followed by a gradual increase through July.

At Grouse ride it follows the exact opposite trend of mountain hemlock, with a gradual increase in correlation strength through May, and then a decrease in June.

The correlation between snowpack and establishment also changes between Scott Paul (fig 14) and

Grouse Ridge (fig 15). At Scott Paul, the correlation decreases in strength between April and May for both mountain hemlock and silver fir. At Grouse Ridge the correlation decreases in strength from April to May for silver fir but increases for mountain hemlock.

Chapter 3

Discussion

3.1 Establishment Pulses

The overall trend is that there was little to no establishment in the late 1800s, followed by the onset of more consistent establishment in the early 1900s. More trees started establishing around Mt. Baker starting around 1926 and ending around 1960. There was then a lull for about 20 years, and then establishment began to increase again from 1981-2015. These pulses coincide with retreats of the Easton glacier, with the glacier response occurring almost a decade after the vegetation response (Whelan and

Bach, 2017). However, the number and exact timing of establishment pulses varied among transects in response to local factors. For example, Scott Paul East only showed the most recent pulse, which we attribute to this meadow having experienced a relatively recent geomorphic disturbance. This is supported by the fact that a significant portion of the meadow vegetation was lupine, which we did not find at any of the other transects. Lupine is typically an early successional species, and its presence would make sense if this meadow were disturbed more recently than the other meadows (Morris and Wood, 1989). The oldest trees at this site were all clustered on a topographically convex area that was not covered in lupine. Another possibility for the single pulse at Scott Paul East is katabatic winds off the Squak Glacier when it was closer during the early 1900s (Whelan and Bach, 2017). Scott Paul East is closer to a glacier than any other transect.

Grouse Ridge East had low levels of establishment and no distinct pulses. There were not enough samples to analyze it on its own, but it still contributes to the overall analysis. It also has the oldest trees of any of the transects, which could indicate different growing conditions here despite its proximity to other sample sites. Other studies in the Washington Cascades have also shown that nearby study sites can have different establishment pulses (Flower et al., 2017; Rochefort and Peterson, 1996, Franklin et al., 1971). Three different studies around Mt. Rainier have all had study sites at Paradise. Each study found subtle difference in the timing of establishment pulses (table 15). While some of this has to do with the timing of the studies and

32 the methods used, microclimates are also a factor in shifting when establishment pulses begin and end. At a coarse spatial scale establishment pulses are driven by climate, but at local scale the influence microclimate and disturbance history is strongly apparent.

3.2 Climate patterns

Rates of seedling establishment are correlated with climatic variability at multiple spatial scales.

For the study area as whole, trees tended to establish during periods with warmer annual temperature, warmer growing season temperature, a greater number of frost-free days and a longer frost-free period, and increased annual precipitation (fig 7). Warmer temperatures lead to increased tree growth, which in turn increases the chances of survival for individual seedlings, and survival rate is a key component of seedling establishment (Lenz et al., 2013; Carins, 2001). Furthermore, increased temperatures lead to a longer growing season, both by having warmer temperatures for a longer period of time and by melting snow at the beginning of the growing season (Flower et al., 2017; Rochefort and Peterson, 1996; Franklin et al., 1971).

It is useful to analyze the climatic patterns at the Scott Paul and Grouse Ridge sites separately due to their different microclimates (fig 2 and 3) and tree responses (fig 9 and 9). Scott Paul is on a south-facing slope and has warmer temperatures than Grouse Ridge, but Scott Paul also gets more precipitation and much more snow. This leads to two different sets of limiting factors on establishment. At Scott Paul, the strongest limiting factor is snowpack in the growing season. Snow covers seedlings, decreasing light exposure and reducing temperatures, both of which decrease rates of growth (Flower et al., 2017; Rochefort and Peterson, 1996; Franklin et al., 1971). The biggest impacts on establishment happen in the months on either end of the growing season. In almost all months at Scott Paul, there is a weak positive correlation between temperature and establishment, which supports the conclusion that establishment is limited by temperature (fig 9). The strongest correlation comes at the end of the growing season in September, when warmer temperatures can extend the growing season and change precipitation to rain instead of snow.

Additionally, there are few consistent patterns between establishment and precipitation at Scott Paul (fig

12). The only consistent pattern is a strong positive correlation between establishment and precipitation in

April, followed by a consistent negative correlation in May. This indicates the importance of the timing of precipitation. Cloudy conditions associated with higher May precipitation decrease the growth rate of trees and in turn shorten the growing season, while precipitation in April occurs before the growing season begins. April precipitation builds up the snowpack and recharges soil moisture, thus ameliorating drought stress over the growing season (Sloat et al., 2015). It is also possible that greater precipitation in April leads to a deeper snowpack, which protects seedlings from dramatic temperature swings and desiccation by wind (Germino and Smith, 1999; Holtmeir and Broll, 2017), but the presence of the same pattern at Grouse

Ridge, which is more moisture limited, indicates that it is more likely related to drought stress. Grouse

Ridge has the opposite pattern of Scott Paul, with inconsistent relationships with temperature (fig 11), and a consistent positive correlations with precipitation, with the exception of May (fig 13). Consistent precipitation and snowpack at Grouse Ridge reduces drought stress in the summer, but too much precipitation in May still reduces the length of the growing season. To summarize, the Scott Paul site is more temperature limited with the strongest positive correlation between establishment and temperature occurring at the end of the growing season in September, and Grouse Ridge is more precipitation limited with the strongest positive correlation between precipitation and establishment occurring in April, just before the start of the growing season.

Scott Paul, which is more snow limited, sees a decrease in correlation strength between establishment and snowpack between April and May (fig 14). This supports the conclusion that snow is limiting establishment at this site. The slight increase in correlation strength from May through July reflects the importance of snowpack to soil moisture. Snowpack will decrease throughout the growing season regardless, and there is a point at the beginning of the growing season where reducing it allows temperatures and exposure to sunlight to increase, but complete snowmelt contributes to drought stress.

This is also supported by the decrease in correlation strength for snowfall (fig 15) from April to May. We did not extend our correlations beyond May, due the extremely limited number of years in which there was snowfall in growing season months beyond May. At Grouse Ridge, the correlations are less consistent.

However, it is still possible to see a decrease in correlation strength between April and May for both snowpack (fig 16) and snowfall (fig 17).

3.3 Spatial patterns

The best scale of analysis for spatial patterns is the transect scale. However, spatial patterns were quite consistent between transects, and the results found at the transect level were also true for the whole study area. There was more establishment on convex surfaces rather than concave surfaces. This is likely due to early snowmelt creating a longer growing season, which leads to increase chances of survival (Flower et. al., 2017; Rochefort and Peterson, 1996). This relationship is well established in the literature but has yet to be thoroughly tested. Future research would benefit from analyzing actual differences in snowmelt timing within individual meadows.

We attempted to see if there was a difference between phyllodoce and vaccinium with regards to establishment, and ended up with mixed results. Both with the data from all the transects combined and separated out, we found that there was a consistent, weak, positive correlation between vaccinium and establishment, but no relationship with phyllodoce. This differs from the findings of Flower et. al, 2017, the only other study to try and make the distinction, which found no relationship between either. Rochefort and Peterson (1996) suggested there may be mycorrhizal associations between vaccinium species and conifers that could facilitate conifer establishment (Cui and Smith, 1990). Meadow soils around Mt. Baker have a very thin organic layer, and it is possible that the loss of leaves from vaccinium species could increase the amount of organic matter in the soil compared to the evergreen phyllodoce species which would not

(Bockheim, 1972). Increased soil carbon would create slightly more favorable establishment conditions in

35 areas with a greater density of vaccinium (Bockheim, 1972). Further study is necessary to determine exactly what differences there may be between phyllodoce and vaccinium cover.

There were also species differences with the correlation between establishment density and distance from seed source. Mountain hemlocks with their light seed had no relationship to seed source distance, while silver fir density was negatively correlated with distance from seed source due to its heavier seed.

This is similar to what was found on Mt. Rainier for mountain hemlock. Silver fir was not a large component of any study on Mt. Rainier, but it has been noted in other studies that Pacific silver fir’s seeds do not disperse as far (Burns and Honkala, 1990). The strong similarities in spatial patterns at all of our transects, as well as in previous studies, further supports the conclusion that the fine scale controls on establishment patterns within a single meadow are fairly consistent between meadows, even if the larger scale average climate conditions at those meadows are different. Trees are most likely to establish on convex surfaces, usually in heather and vaccinium which can facilitate growth, and silver fir is particularly limited in the distance that it can disperse.

3.4 Age patterns

We found no significant spatial relationships for the ages of trees. We analyzed both the difference in ages of trees on convex and concave plots (fig 19), as well as how age correlated to distance from seed source (fig 21). This indicates that when trees establish in pulses, they are gradually filling in all suitable areas over time, such as convex areas with vaccinium coverage, rather than first filling in the most suitable areas for establishment prior to establishing in slightly less suitable microsites. This is slightly different from the findings of Flower et al., 2017, which is the only other study in the region to have analyzed spatial age patterns of subalpine conifer encroachment. They found that trees in convex plots were significantly older than trees in concave plots. At Mt. Baker, we found trees were consistently older on convex plots, but the relationship was not statistically significant. The difference in the findings between the two studies

36 indicates that there are slight differences in spatiotemporal availability of habitat for establishment between

Mt. Baker and Mt. Rainier. At both Mt. Rainier and Mt. Baker, convex surfaces became suitable for the onset of establishment earlier in the 20th century than concave surfaces, but the relationship is stronger at Mt.

Rainier.

We also found significant positive correlations between basal diameter and age, as well as height and age. Flower et al., 2017 found only a significant positive relationship between height and age. At Mt.

Rainier it was more beneficial for seedlings to put more of their resources into growing vertically, most likely to have more of their needles exposed to warmth and sunlight above the snowpack in the early growing season. At Mt. Baker the seedlings put as many resources into growing vertically as they did into increasing in width, with similar correlations between height and age, and basal diameter and age (fig. 22, table 13). Most trees were growing vertically with one single stem, except for yellow cedar which occasionally grew vertically with multiple stems. None of the samples came from Krumholtz trees, as there were none in our transects. According to data from a SNOTEL site reported by the Natural Resource

Conservation Service, early season snowpack at Paradise is consistently deeper than it is at Mt. Baker, which would account for the differences in growth patterns (fig 23).

Figure 23. A comparison of early growing season snowpack at Mt. Baker Transects and at Paradise on the south side of Mt. Rainier.

3.5 Species patterns

Each species has a different response to climate and microclimates. Yellow cedar, which is more of a generalist, is the most drought tolerant of the trees in the study area (Laroque and Smith, 1999; Antos and

Zobel, 1986). The presence of yellow cedar at Grouse Ridge immediately indicates that Grouse Ridge is a drier site than Scott Paul, which is supported by climate data (fig 2). The most notable difference between yellow cedar and the other two species is its response to snowpack (fig 15) and snowfall (fig 17). It has a far stronger association with snowpack, as well as snowfall in April. As yellow cedar is the more drought tolerant tree, this indicates that within the meadows yellow cedar is more likely to establish on microsites that experience higher moisture stress due to soil and microtopographic conditions. This allows yellow cedar to avoid competition with mountain hemlock and silver fir, but makes it more reliant on snowpack to mitigate microsite conditions.

3.6 Regional patterns

There are general patterns in the timing of conifer encroachment throughout the Pacific Northwest.

These patterns are driven by regional climatic changes, as well as differences in aspect, microclimate, and tree species present between sites. The general trend for the region as a whole is a nearly continuous period of establishment from early 1920s through the 1960s, with pulses in the early 1930s and early 1940s (fig 24).

This period of establishment is followed by another pulse, starting in the 1980s and ending in the early

1990s. A pulse, sometimes also referred to as a peak, has neither a precise nor consistent definition, but is generally used to refer to a period of time when there is greater than normal levels of seedling establishment.

Figure 24. Pulses of conifer establishment using nine different studies in the Pacific Northwest.

Our research at Mt. Baker fits well within the regional patterns of conifer encroachment, while also showing some key location-specific differences. The only previous study on forest expansion in the subalpine zone at Mt. Baker (Heikkinen, 1984) showed that there was a correlation between periods of establishment and periods of higher temperatures and lower precipitation. The author also noted that there was a correlation between aspect and establishment, with aspects that receive more solar radiation having higher rates of establishment (Heikkinen, 1984). In the Pacific Northwest, conifer encroachment has been

39 most frequently studied around Mt. Rainer. Mt. Rainier has a variety of microclimates in the subalpine zone due to changes in precipitation around the mountain (Rochefort and Peterson, 1996). With wind direction and orographic uplift, the west side of the mountain has greater precipitation and deeper snowpack than the east side. This difference in climate creates conditions favorable to different tree species at various times, resulting in different temporal patterns of establishment. On the south and west sides of

Mt. Rainier, establishment increased when summer temperatures were warmer, summer snowfall lower, and PDSI was higher (Franklin et al., 1971; Rochefort and Peterson, 1996; Flower et al 2017). While on the northeast side, encroachment increased during periods of lower summer temperatures, increased summer precipitation, and increased PDSI (Rochefort and Peterson, 1996). Similarly, at our sites on Mt Baker, establishment at our drier site was more strongly correlated with moisture availability, and establishment at our site with heavier snowpack is more strongly correlated with temperature. Unexpectedly, our dry site is on a north facing slope. There is likely a localized rain shadow that causes a dramatic difference in precipitation between Scott Paul and Grouse Ridge. Twin Sisters Mountain, a lightly glacier-clad peak just to the southwest of Mt. Baker and south of Grouse Ridge, has two summits with elevations of 2,135 and

2,025 meters, which is over 400 meters above the highest point on Grouse Ridge. Wind direction in this area, particularly during winter months with high precipitation, is generally southerly (Kistler et al., 2001).

This could lead to Grouse Ridge being in the small rain shadow of Twin Sisters, while Scott Paul is not.

Results from Studies at Paradise Flower et. al., 2017 Rochefort and Franklin et. al. 1971 Peterson 1996 Establishment Pulses 1936-1970 1935- 1950, 1955-1960, 1928-1937 1980-1985 Microtopography Convex (+) Convex (+) Convex (+) Distance to Seed Source Not assessed No significant No significant correlation correlation Vegetation No significant Greater in Heather Greater in Heather correlation and Vaccinium and Vaccinium

Climate Variables Mean annual and May, June, July Temp PDSI, July Temp (+) growing season (+) Temp (+), Mean summer snowfall (-) Table 14. Reported relationships between establishment and climatic and spatial variables in three previously published studies conducted at Paradise at Mt. Rainier

Our individual transects at Mt. Baker showed varying timing of establishment pulses, even at nearby transects, but consistent relationships between establishment density and spatial variables. This pattern has also been observed at Mt. Rainier. Paradise, a popular tourist destination on the south side of Mt.

Rainier, has been studied more frequently than other locations around the mountain. Despite the fact that all sites at Paradise experience similar climates and have similar assemblages of tree species, and encroachment at these sites has a fairly similar relationship to changes in climate, each study found different periods of time with pulses of establishment (Franklin et al., 1971; Rochefort and Peterson, 1996;

Flower et al 2017). This is likely due to microclimatic differences between sites caused by local variability in factors such as slope aspect (Flower et al., 2017). While the temporal patterns on different sides of a mountain and even at similar sites might be variable, the spatial patterns of where trees are establishing in the meadows is consistent across all locations (Table 14). Trees tend to establish more densely on microtopographic convexities as well as within the areas covered with phyllodoce/vaccinium community, and with no significant relationship to the distance from a seed source (Franklin et al., 1971; Rochefort and

Peterson, 1996; Flower et al 2017). These are generally the same relationships we observed at Mt. Baker.

While there may be regional and landscape scale differences in climate, microclimates within meadows tend to create areas that are more favorable for establishment, and the characteristics of favorable microclimates in meadows is consistent throughout the region.

Similar temporal and spatial patterns are also found outside the Washington Cascades in similar climatic regions. In the Olympic Mountains, mountain hemlock established more frequently on the wetter end of a precipitation gradient during drier periods in the 20th century, while on the drier end of the gradient subalpine fir established more frequently during wetter portions of the 20th century (Woodward et al., 1995). There is also denser establishment in phyllodoce/vaccinium cover than in other sorts of vegetation cover. On southern Vancouver Island mountain hemlock establishment was greatest during periods with low snowpack and normal summer temperatures, or warmer than normal summers with moderately deep snowpack (LaRoque et al., 2000). This fits with the pattern on the west side of Mt. Rainier, the Olympics, and at Scott Paul where conifer establishment is more successful when snow melts fast enough to increase the growing season, but not so much that it creates drought conditions. Silver fir and subalpine fir on Vancouver Island established more during cool growing seasons with moderately deep snowpack, but likely for different reasons, as subalpine fir tends to grow in more drought prone microsites and silver fir grows in mesic sites but is extremely drought intolerant (LaRoque et al., 2000).

There are similar divides in temporal patterns of establishment in Oregon, but instead of a west to east precipitation gradient they found a difference between north and south facing slopes due to changes in radiation (Miller and Halpern, 1998). South-facing slopes showed similar patterns of establishment to sites on the east side of Mt. Rainier with establishment occurring during wetter periods, while north facing slope were more like the west side. Spatially, in Oregon they found that within meadows the type of landform on which establishment is occurring influences how densely and trees are establishing. The landform modifies the depth of the snowpack, which in turn changes the length of the growing season, which is consistent with the regional pattern (Zald et al., 2012). At Mt. Baker the south facing slopes were more like

42 the west side of Mt. Rainier, and the north facing slope more like the east side of Mt. Rainier. This is the exact opposite of the pattern found in Oregon, and is so far unique to Mt. Baker. However, both Oregon and Mt. Baker showed similar spatial patterns of establishment, with landforms in meadows providing more or less suitable habitat depending on microclimatic conditions.

3.7 Future Research

There are several ways to improve on existing conifer encroachment research in the Pacific Northwest.

The first is researching causal links between encroachment and consistently correlated variables. Studies have consistently shown that there is denser establishment on convex surfaces, and most have concluded it is due to earlier snowmelt. This relationship should be examined in greater detail. We do not yet know exactly how much sooner snow melts from these convex surfaces, how spatially variable the timing of snowmelt is within a single meadow, or how spatial variability of snowmelt within a meadow changes over time. Answering these questions would allow us to understand exactly how much encroachment is related to snowmelt. We also largely ignored soil conditions in this study, and some of our results indicated that studying soil composition is necessary to answer further questions. Our results indicated that there is a stronger relationship between establishment and vaccinium cover than there is with phyllodoce cover.

Previous papers tended to group these plant families together, which makes sense considering the spatial cooccurrence. By separating them we found a novel result that needs to be investigated. Soil nutrients and soil moisture surly play a role in shaping patterns of conifer encroachment, and understanding what role they play will help us to better understand the patterns we observe.

Research should be expanded into more areas of the Cascades. While we can clearly see an overall regional pattern, there are also distinct differences. Mt. Baker and Mt. Rainier are still largely similar to each other, and not necessarily representative of rest of the Cascades. Further exploration will lead to research with different microclimates, different geologic origins, soil conditions, and unique assemblages

43 of trees. These changes could lead to a better understanding of the true spatial and temporal variability of encroachment in the region.

Indigenous plant cultivation and the impact it has on spatial patterns is frequently understudied in ecological research (Duer, 2002). Indigenous peoples along the Northwest Coast burned subalpine meadows as a management strategy for berry production (Lepofsky and Lertzman, 2008). While we did not see evidence of fires during our data collection, it is not uncommon for this sort of anthropogenic signal to be challenging to separate from other fire regimes, even in areas where this sort of cultivation is known to have occurred (Lepofsky and Lertzman, 2008). The legacy of indigenous cultivation in some areas rather than others is another aspect that could alter the spatial and temporal patterns of conifer encroachment.

Research must be conducted with Tribes and First Nations groups to fully grasp the changes happening along the subalpine ecotone.

Chapter 4

Conclusion

We investigated conifer encroachment into subalpine meadows on Mt. Baker using dendrochronological methods. We set out to reconstruct the local history of conifer seedling establishment and quantify the spatiotemporal relationships between establishment and a suite of climatic, ecological, and topographic variables. We collected cross sections and counted whorls from mountain hemlock, silver fir, and yellow cedar along a total of four transects, two on a north facing slope and two on a south facing slope. We also measured the height and basal diameter of trees and seedlings, and collected data on distance from seed source, vegetation cover, and microtopographic position. We measured, counted the rings, and crossdated our cross sections, and used the ring counts along with our whorl counts to get ages for our samples, which we grouped into five-year bins.

The timing of establishment pulses, during which many conifer seedlings successfully established over a short time period, was broadly similar to the dates reported for other mountains in the region. We found the onset of consistent establishment began in early 1900s, and that there were two major pulses of establishment: 1926-1930 through 1956-1960 and 1981-1985 through 2011-2015. With the ages of our samples, we ran partial correlations with numerous annual, seasonal, and monthly climate variables. We found that across our study area, establishment was correlated with warmer temperatures and a longer growing season. At the site level, establishment at our south facing site was primarily limited by growing season length and was therefore positively correlated with temperature. Establishment at our north facing site was more moisture limited, and therefore positively correlated with precipitation. The timing of the precipitation and temperature changes is important, with increased snowpack in April leading to greater establishment, but increased snowpack in May leading to decreased establishment. This indicates that there is more establishment when snowpack is deep enough to prevent drought stress in the summer, but not so deep that trees are covered at the beginning of the growing season. Similarly, September is the month that

45 had the strongest correlation between temperature and establishment, showing the importance of a prolonged growing season. At our moisture limited site, we had yellow cedar, which showed a particularly strong positive correlation with establishment and monthly snowpack variables. Yellow cedar is the most drought tolerant of the species present, which indicates that yellow cedar is establishing on the microsites most prone to drought stress and therefore most responsive to soil moisture from snowpack.

Seedlings established in favorable spots within each meadow, which lead to patchy patterns of encroachment. Establishment density was higher in areas with convex microtopography and lower in areas with concave microtopography. It was also higher in areas with denser vaccinium coverage, but not phyllodoce coverage. Distance from seed source only affected silver fir, which has a heavier seed and does not disperse as far into the meadows. Trees on convex surfaces were older than trees on concave surfaces, which supports the conclusion that snowpack is limiting seedling establishment. These areas melt out both sooner in the year, and became potential establishment sites sooner in the 20th century, making them the first areas suitable for establishment. Both basal diameter and height were well correlated with tree age.

The subalpine ecotone, like many ecosystems, is already changing in response to climate change

(Grace et al., 2002; Holtmeir and Broll, 2007). Conifer encroachment is reducing critical meadow habitat, making it more challenging for species that either live solely in that habitat or depend on it for food or shelter. The addition of trees is altering the hydrologic cycle, which will influence changes in areas far away from the meadows themselves. Mt. Baker is not immune to these changes. In areas that are more limited by snow, conifers will encroach into previously treeless areas during periods of time that are warmer and drier, particularly in the growing season. Growing seasons are predicted to become longer, warmer, and drier as climate change continues over the 21st century (Dickerson-Lange and Mitchell, 2014), which will likely lead to increased rates of conifer encroachment into many subalpine meadows. Other areas are more moisture limited and may see decreased establishment during those periods due to drought stress. This results in spatial variability in habitat loss, leaving species in some areas with less habitat and generally

46 reducing habitat connectivity. It also means that some watersheds will begin to see changes in summer stream flows before others will. Some meadows are also more suitable for establishment than others due to the presence of convex surfaces and increased vaccinium cover, which will increase the spatial variability of habitat loss. The forest that takes the place of the meadows likely will look different than the forest that exists currently. Silver fir will have an easier time establishing than before, creating bands of young silver fir near the well-established silver fir on the convex edges of meadows. More microsites are likely to become drought stressed, increasing the number of suitable microsites for yellow cedar to establish. While these are the likely outcomes at Mt. Baker, it is unlikely that Mt. Baker is representative of the North Cascades as a whole. Mt. Baker does share the overall regional trends in timing of conifer establishment reported for other areas in previous studies, but there are enough differences, even between sites at the mountain itself, that it is clear that research must be conducted more widely.

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Appendix A

Figure A1 shows the correlations between different climate variables.

Climate Variable t-score Degrees of p-value R Correlations Freedom

MAT and MAP -0.165 108 0.869 -0.016

May Temp and Precip -2.556 108 0.012 -2.388

September Temp and Precip -5.537 108 2.182e-07 -0.470

May Precip and Snowpack -1.543 108 0.1257 -0.147

May Temp and Snowpack -2.58 108 0.011 -0.241

Table A1 shows the values for the correlations shown in fig A1

Appendix B

Shapiro Test p- values Phylo. Vacc. Lupinus TSME ABAM CANO All Trees GRW18 0.001 5.73E-10 NA <2.2e-16 1.52E-15 < 2.2e- 1.72E-12 16 GRE18 3.87E-08 6.61E-10 NA <2.2e-16 < 2.2e-16 < 2.2e- 2.80E-15 16 SPW18 3.68E-06 2.88E-05 NA 1.35E-12 < 2.2e-16 NA 9.64E-11 SPE18 3.98E-05 6.00E-10 4.99E-11 1.45E-12 < 2.2e-16 NA 4.27E-12 Table B1 shows consistently significant p-values for the Shapiro-Wilkes test, which indicates that data is not normally distributed.

Fligner Test p- values Phylo. Vacc. Lupinus TSME ABAM CANO All Trees 0.007 1.02E- < 2.2e-16 7.01E-07 1.25E- 3.98E-10 0.0001 10 08 Table B2 Shows consistently significant p-values for the Fligner-Killeen test, which indicates that data is not homoscedastic.

Figure B1 shows the difference between the transformed and untransformed residuals for linear models built to show the relationships between age and growth of trees.

Appendix C

Figure C1. Normal Kendall’s correlations between establishment and annual and seasonal climate variables at Scott Paul. *=p-value < 0.05, **=p-value < 0.01. Mean Annual Temperature (MAT), Mean Growing Season Temperature (MGST), Mean Annual Precipitation (MAP), Mean Growing Season Precipitation (MGSP), Mean Water Year Temperature (MWYT), Mean Water Year Precipitation (MWYP), Frost Free Period (FFP), and Number of Frost Free Days (NFFD).

Figure C2. Normal Kendall’s correlations between establishment and annual and seasonal climate variables at Grouse Ridge. *=p-value < 0.05, **=p-value < 0.01. Mean Annual Temperature (MAT), Mean Growing Season Temperature (MGST), Mean Annual Precipitation (MAP), Mean Growing Season Precipitation (MGSP), Mean Water Year Temperature (MWYT), Mean Water Year Precipitation (MWYP), Frost Free Period (FFP), and Number of Frost Free Days (NFFD).

Figure C3. Normal Kendall’s correlation between mean monthly temperature and establishment at the Scott Paul site. *=p-value < 0.05, **=p-value < 0.01.

Figure C4. Normal Kendall’s correlation between mean monthly temperature and establishment at the Grouse Ridge site*=p-value < 0.05, **=p-value < 0.01

Figure C5. Normal Kendall’s correlation between mean monthly precipitation and establishment at the Scott Paul site. *=p-value < 0.05.

Figure C6. Normal Kendall’s correlation between mean monthly precipitation and establishment at the Grouse Ridge site.. *=p-value < 0.05.

Figure C7. Normal Kendall’s correlation controlled for temperature between mean monthly snowpack and establishment at Scott Paul. None of these correlations are significant.

Figure C8. Normal Kendall’s correlation controlled for temperature between snowpack and establishment at Grouse Ridge. *=p-value < 0.05. July is not included for snowpack at Grouse Ridge because there is rarely any snowpack at Grouse Ridge in July.

Figure C9. Normal Kendall’s correlation controlled for temperature between snowfall and establishment at Scott Paul. *=p-value < 0.05.

Figure C10. Normal Kendall’s correlation controlled for temperature between snowfall and establishment at Grouse Ridge. *=p-value < 0.05.